Abstract

Ocean acidification (OA) is a major component of ongoing global environmental change, yet its biological impacts on open-ocean calcifiers remain insufficiently quantified. Here, we investigate seasonal variability in the individual test density of the planktic foraminifer Globigerina bulloides in the western North Pacific. Test density was determined using high-resolution microfocus X-ray computed tomography, enabling micron-scale structural assessment. Time-series samples collected by sediment traps moored at 150 m and 540 m at station K2 (2008–2009) reveal pronounced seasonal variability, with test density reduced by ~ 20% during winter relative to other seasons. Seasonal reductions were associated with enhanced vertical mixing and positively correlated with mixed-layer pH, carbonate-ion concentration, and temperature. Additional plankton-tow samples collected between 2010 and 2016 further support a strong linkage between carbonate chemistry and calcification intensity. Multiple regression analysis shows that carbonate-ion concentration independently explains 46.6% of the variance in test density, whereas temperature accounts for only 0.25%, indicating that carbonate-ion availability exerts a dominant control on test density. Given the ongoing decline in carbonate-ion concentration in the North Pacific (~ 0.77 µmol kg⁻¹ yr⁻¹), our results imply an annual decrease of ~ 2 µg mm⁻³ in foraminiferal test density. Continued OA may therefore reduce biogenic CaCO₃ shell density, potentially weakening the efficiency of the carbonate-based biological carbon pump. This study provides quantitative field-based evidence linking seasonal carbonate chemistry to shell density variability in open-ocean calcifiers.